1. Field of the Invention
The present invention relates to a wavelength division multiplexing (WDM) transmission system and a remote apparatus and a station apparatus used therein and, in particular, to a wavelength division multiplexing transmission system in which a plurality of remote apparatuses are connected to a station apparatus in a star or tree topology, and the remote apparatuses and the station apparatus used therein.
2. Description of the Related Art
The widespread use of the Internet has brought about the explosion in demand for data communications. WDM transmission systems are used in order to efficiently increase communication traffic.
Examples of network system configurations include a star system configuration in which a number of remote apparatuses are connected to one station apparatus and a tree system configuration in which a number of remote apparatuses are connected to each other through a relay point such as a star coupler.
FIG. 1 shows a block diagram of a prior-art wavelength multiplexing transmission systems. Referring to FIG. 1, a wavelength division multiplexing transmission system includes a station apparatus 10, a plurality of remote apparatuses 20-1-20-n (wherein n is a positive integer), a wavelength demultiplexer 7 connected between the station apparatus 10 and the plurality of remote apparatuses 20-1-20-n, and a wavelength multiplexer 8. In this way, the station apparatus 10 and the plurality of remote apparatuses 20-1-20-n are connected to each other through the wavelength demultiplexer 7 and the wavelength multiplexer 8 in a star topology.
The station apparatus 10 includes a plurality of optical transmitters 101-10n a plurality of optical receivers 111-11n, a wavelength multiplexer 3 which outputs signals 1011 to 10n1 provided from the optical transmitters 101-10n as wavelength multiplexed optical signals, and a wavelength demultiplexer 4 which demultiplexes a wavelength multiplexed optical signal 2020 and outputs the resulting signals to the optical receivers 111 to 11n. The wavelength demultiplexer 7 demultiplexes a wavelength multiplexed optical signal 2010 provided from the wavelength multiplexer 3 of the station apparatus 10 and outputs the resulting signals. The wavelength multiplexer 8 multiplexes individual optical signals 2021-202n and outputs the resulting signal to the wavelength demultiplexer 4 of the station apparatus 10. A remote apparatus 20-1 includes an optical receiver 220-1 which receives an optical signal 2011 among signals provided from the wavelength demultiplexer 7, an optical transmitter 230-1 which outputs each individual optical signal 2021 to the wavelength demultiplexer 8, and wavelength controller 240-1 which controls the wavelengths of optical signals to be transmitted from the optical transmitter 230-1. The configurations of remote apparatuses 20-2-20-n, which are not shown, are similar to the configuration of the remote apparatus 20-1. Each of the remote apparatuses has an optical receiver 220-2-220-n which receives a signal 2012-201n among signals provided from the wavelength demultiplexer 7, an optical transmitter 230-2-230-n which transmits each individual optical signal 2022-202n to the wavelength multiplexer 8, and a wavelength controller 240-2-240-n which controls the wavelengths of optical signals to be transmitted from the optical transmitter 230-2-230-n. 
Shown in FIG. 1 is the addition of a remote apparatus 20-m (m is any positive integer within the range from 1 to n) to the prior-art wavelength division multiplexing transmission system. The optical transmitter 230-m of the newly added remote apparatus 20-m contains a wavelength tunable laser. The output wavelength of the wavelength tunable laser must be controlled so as to be the wavelength assigned to the remote apparatus 20-m through the use of a wavelength controller.
Another example of wavelength division multiplexing transmission systems of this type is described in Japanese Patent Laid-Open No. 9-83491 (paragraph 0016 and FIG. 2, hereinafter referred to as a first patent document), which is a wavelength division multiplexing communication network on which a line monitoring node is provided. The line monitoring node searches through wavelengths on the network. If the line monitoring node detects an unused wavelength, it transmits a signal indicating that the wavelength is unused. If the line monitoring node later detects in that wavelength a signal other than the signal indicating that the wavelength is unused, it ends the transmission of the signal indicating that the wavelength is unused and searches for another unused wavelength on the network.
However, in the prior-art system shown in FIG. 1 and described above, each time a new remote apparatus is installed, a wavelength to be used in that system must be set by a maintainer or other personnel. Therefore, the system has a problem in that man-hours for such maintenance tasks are required and it takes time for a new remote apparatus to start operation.
The technology described in the first patent document requires a dedicated line monitoring node for detecting an unused wavelength. Accordingly, it requires an arrangement and operation for notifying a remote node (which is equivalent to a remote apparatus of the prior art herein) of the unused wavelength detected by the line monitoring node. Furthermore, because bidirectional communication relating to information about unused wavelengths must be performed between remote nodes, collisions between signals may occur and action must be taken to handle them. As described above, the technology disclosed in the first patent document has a problem in that the technology requires a complex arrangement, operation, and processing.